Add graphs and retention metrics to admin dashboard (#16829)

app/controllers/api/v1/admin/retention_controller.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class Api::V1::Admin::RetentionController < Api::BaseController
+  protect_from_forgery with: :exception
+
+  before_action :require_staff!
+  before_action :set_cohorts
+
+  def create
+    render json: @cohorts, each_serializer: REST::Admin::CohortSerializer
+  end
+
+  private
+
+  def set_cohorts
+    @cohorts = Admin::Metrics::Retention.new(
+      params[:start_at],
+      params[:end_at],
+      params[:frequency]
+    ).cohorts
+  end
+end